METHODS AND APPARATUS FOR CONNECTING AND CONDITIONING AUDIO SIGNALS

Abstract: Audio signal connection and conditioning circuitry employs a resistive drop across the ground input for the signal source to generate a common mode voltage on both input lines. A differential amplifier substracts the generated common mode voltage on the ground line from the common mode voltage on the source input line, leaving only the source voltage as an output. The differential amplifier output is generated with respect to a positive, stable voltage reference instead of a floating ground.

Description: METHODS AND APPARATUS FOR CONNECTING AND CONDITIONING AUDIO SIGNALS 1. CROSS-REFERENCE TO RELATED PATENT APPLICATIONS This patent application is related to the patent application entitled "Audio Peripheral Mixer Circuit and Method for Noise Reduction" by David Franklin Wilson and Lawrence Frederick Heyl, filed May 13, 1993, Serial Number 08/062,422 and assigned to Apple Computer, Inc., the disclosure of which is hereby incorporated by reference. 2. FIELD OF THE INVENTION The present invention relates to electronic amplifier systems. In particular the invention relates to connecting a source of audio signals to a computer system. 3. BACKGROUND OF THE INVENTION With increasing sophistication, personal computers interact with users through a variety of multi-media apparatus and techniques. In particular, desktop computers have begun to incorporate signals from a variety of sound sources, including microphones, tape and CD players, video inputs and telephone equipment. These sources provide sound and voice information for use by computer applications. For instance, sampling program applications can capture, store and modify voice and music information. As another example, speech recognition and synthesis programs can parse human speech and perform program commands based upon the recognized human voice. New uses for sound inputs will continue to expand computer performance. Unfortunately, several factors hamper the use of these sound inputs in desktop computers. The desktop computer, designed for digital purposes, presents a rather hostile environment for audio frequency electronic operation. Desktop computers continuously generate digital signals having spurious electromagnetic frequency components in the audio region, creating common mode noise. In addition, series inductance and series resistance drops (IR drops) within circuit boards, connectors and cables exacerbate this common mode noise. Power mains frequencies and their principal harmonics also add to this common mode noise. Common mode noise makes connecting two analog devices together (such as a sound input and a desktop computer on-board preamplifier) difficult. The straightforward approach connects the ground signals of the two devices together and then connects the external signal source to a suitable input point. However, because of common mode noise, ground is seldom true "ground". A floating ground, caused by coupled common mode noise, can interfere with and degrade voice and sound signals. Exposed to this interference, program applications can exhibit anomalous and often erroneous behavior. Speech recognition applications, in particular, require fairly high signal-to-noise ratios to extract recognizable words from a stream of sound data. Common mode noise degradation can hamper and even prevent proper operation of these human voice and sound applications, especially from sound sources that provide very low level signals. Two conventional methods can be tried for connecting audio signals to a desktop computer. One simple method uses a single-ended connection, thereby connecting grounds together and simply hoping that the differences in grounds will not be significant. The other method employs complex and expensive full differential inputs to minimize common mode noise. Unfortunately, the computer industry faces significant competitive pressures to reduce costs and increase reliability and manufacturing yields. Expensive or unreliable techniques for handling common mode noise are simply not viable for most desktop computer applications. Current apparatus and methods for connecting analog components together do not provide a simple, inexpensive approach for connecting sound inputs to a desktop computer system while minimizing floating ground and noise degradation. These conventional approaches do not provide a high-performance sound input connection and conditioning circuit that is easy to manufacture, with high yields and dependable performance. What is needed is an improved method and apparatus for connecting and conditioning audio signals for computer applications. An improved apparatus and method for connecting and conditioning audio signals should provide a simple, inexpensive and dependable approach for the inputting of sound and human voice signals. The improved apparatus and method should provide a flexible approach that easily adapts to any particular desktop computer environment, and that can be incorporated in computer circuitry with inexpensive, readily available parts. 4. SUMMARY OF THE INVENTION In accordance with the present invention, a connecting and conditioning system for audio signals comprises an input for an audio signal, one or more resistive drops connected to the input, and a differential amplifier connected across the input, where the differential amplifier provides a conditioned audio signal output referred to a stable, quiet voltage reference. A first resistive drop from one line of the input generates a common mode voltage, or VQM across both lines of the input. A second resistive drop across both lines of the input generates the actual input signal voltage drop, or Vg. The differential amplifier serves to subtract out the common mode voltage signal V^ from both input lines, leaving just V s , referred to a stable voltage reference V re f, instead of a possibly noise-corrupted ground. With only a few active components available off-the-shelf, the unique design of the connection and conditioning system of the present invention provides inexpensive methods and apparatus for connecting sound signals to a desktop computer, or to any other electronic application requiring low-cost, high-performance audio signal connection and conditioning. The present invention can connect two audio components, significantly reduce common mode noise inherent to such electronic devices and couplings, and provide useful audio connections for desktop computer applications. The output signal of the present invention is a clean signal voltage, significantly free of any common mode voltage noise. These and other features and advantages of the present invention are apparent from the description below with reference to the following drawing. 5. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic diagram of an audio signal connection and conditioning circuit in accordance with the present invention. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, an audio signal connection and conditioning circuit 10 is shown in FIG. 1. The circuit includes a signal source 12, one or more resistive loads 14, and a differential amplifier block 16. The signal source 12 comprises a source signal 18 (V s ) and a source resistance 20 (R s ). The output of the signal source attaches to input lines 21a and 21b. In a conventional audio signal input, a variety of sources would contribute to generate a net common mode voltage noise source across the input lines, represented by V CM or noise source 22a. The sources of common mode noise include power mains frequencies at 50 or 60 Hz; the first few significant harmonics of power mains frequencies (i.e., 100, 150, 200, 250 Hz or 120, 160, 240 and 300 Hz); noise created by the harmonics of digital signals; power supply noise; and any other noise inherent to the input signal source V s . The present invention significantly reduces any such common mode noise across inputs 21a and 21b. First, the present invention generates a second VCM across input line 21b by using resistive drop 24. Resistor 24 in the preferred embodiment is 49.9 ohms. A second resistive drop 26 across input lines 21a and 21b is 10 Kohms. By choosing resistor 24 to be greater than competing return paths on a typical circuit board implementation, V Q ^ is generated on both sides of resistor 26, while V s is generated only on input line 21a. The differential amplifier, discussed further below, can then take the signal on line 21a (or V s +V CM )and subtract the signal found now on line 21b (or just ~ V~ M) t0 Y^ me pure audio signal Vg. Differential amplifier 16 subtracts the generated signals on input lines 21a and 21b. The core of differential amplifier 16 is operational amplifier 32, which can be an LMC660C or MC34074 or similar device. Input lines 21a and 21b are coupled to the inputs of operational amplifier 32 by two pairs of capacitors and resistors, 28a and 30a and 28b and 30b. Capacitors 28a and 28b (1.0 μF) block the reference voltage V Ref (one input 42b of the differential amplifier block 16), providing a unity DC gain for circuit 10. Resistors 30a and 30b were chosen to be much greater than the typical external source impedance R $ (Resistance 20). Assuming a maximum R $ of 1000 ohms (where typically R s will be under 100 ohms), a value of 44 Kohm for resistors 30a and 30b represents a good trade-off between negligible loading of the signal input 12 and low noise contribution to circuit 10. Resistors 30a and 30b and capacitors 28a and 28b also establish the lower cut-off frequency for the amplifier. In the preferred embodiment, this cut-off frequency was chosen to be a quite low 3.6 Hz so that common mode noise rejection would be adequate over the audio band, but especially at the power line frequencies of 50 Hz and 60Hz. Setting the lower cut-off frequency also allows for much looser tolerance parts to be used in production, increasing manufacturing yields. Resistors 30a and 30b. in conjunction with resistors 36a and 36b (22 Koh-ms), set the AC gain of the differential amplifier stage, simply defined as: Thus, given the values of the preferred embodiment, circuit 10 has an AC gain of -0.5, and a DC gain of 1.0. Output 38a of the differential amplifier 16 provides V s , while mput 38b is provided a stable, quiet +2.5 V. reference, instead of an unstable, floating ground. In addition, resistors 30a, 30b, 36a and 36b are constructed as a resistor network, for stability of circuit operation combined with an economy of parts. Also, since resistor 26 is much greater than either resistance 20 (Rs) or resistor 24, resistors 24 and 26 have a negligible effect on the AC gain of circuit 10. When Rg tends towards 0 ohms, circuit 10 will achieve up to 54 dB rejection of common mode noise. With R $ = 1000 ohms, common mode rejection degrades to 40 dB. Capacitors 34a and 34b roll off the high frequency response of operational amplifier 32, such that with a common mode rejection of -3dB at 27 kHz, the common mode rejection of circuit 10 stays relatively flat within ± 2dB from 20 Hz to 20 kHz. While the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that various modifications may be provided. A number of equivalent apparatus can be used to implement the present invention. For example, any number of equivalent components can be substituted for the operational amplifier, capacitors and resistors. These and other variations upon and modifications to the described embodiments are provided for by the present invention, the scope of which is limited only by the following claims.

Claims: CLAIMS What Is Claimed Is: 1. Apparatus for connecting and conditioning an audio signal comprising: an audio signal input for the audio signal; a first input resistive drop on one line of the audio signal input; and a differential amplifier coupled across the audio signal input, the differential amplifier having an output and a voltage reference input. 2. The apparatus as claimed in Claim 1 further comprising: a second input resistive drop across two lines of the audio signal input. 3. The apparatus as claimed in Claim 1 further comprising: a first resistor and a first capacitor coupling a first line of the audio signal input to a first input of the differential amplifier; and a second resistor and a second capacitor coupling a second line of the audio signal input to a second input of the differential amplifier. 4. The apparatus as claimed in Claim 1 further comprising: a third resistor and a third capacitor coupling the output of the differential amplifier to a first input of the differential amplifier. 5. The apparatus as claimed in Claim 1 further comprising: a fourth resistor and a fourth capacitor coupling the voltage reference input of the differential amplifier to a second input of the differential amplifier. 6. Apparatus for connecting and conditioning an audio signal comprising: an audio signal input for the audio signal; an input resistive drop on one line of the audio signal input; a differential amplifier coupled across the audio signal input, the differential amplifier having an output and a voltage reference input; a first resistor and first capacitor coupling a first line of the audio signal input to a first input of the differential amplifier; a second resistor and a second capacitor coupling a second line of the audio signal input to a second input of the differential amplifier; a third resistor and a third capacitor coupling the output of the differential amplifier to the first input of the differential amplifier; and a fourth resistor and a fourth capacitor coupling the voltage reference input of the differential amplifier to the second input of the differential amplifier. 7. Apparatus for connecting and conditioning an audio signal comprising: an audio signal input means for inputting the audio signal; a first input resistive drop means for generating a voltage drop on one line of the audio signal input means; and a differential amplifier means coupled across the audio signal input means for subtracting out common mode noise from the audio signal, the differential amplifier having an output and a voltage reference input. 8. The apparatus as claimed in Claim 7 further comprising: a second input resistive drop means for generating a voltage drop across two lines of the audio signal input means. 9. The apparatus as claimed in Claim 7 further comprising: a first resistor means and a first capacitor means for coupling a first line of the audio signal input means to a first input of the differential amplifier means; and a second resistor means and a second capacitor means for coupling a second line of the audio signal input means to a second input of the differential amplifier means. 10. The apparatus as claimed in Claim 7 further comprising: a third resistor means and a third capacitor means for coupling the output of the differential amplifier means to a first input of the differential amplifier means. 1 1. The apparatus as claimed in Claim 7 further comprising: a fourth resistor means and a fourth capacitor means for coupling the voltage reference input of the differential amplifier means to a second input of the differential amplifier means. 12. Apparatus for connecting and conditioning an audio signal comprising: an audio signal input means for inputting the audio signal; an input resistive drop means for generating a voltage drop on one line of the audio signal input means; a differential amplifier means coupled across the audio signal input means for subtracting out common mode noise from the audio signal, the differential amplifier having an output and a voltage reference; a first resistor means and first capacitor means coupling a first line of the audio signal input means to a first input of the differential amplifier means; a second resistor means and a second capacitor means coupling a second line of the audio signal input means to a second input of the differential amplifier means; a third resistor means and a third capacitor means coupling the output of the differential amplifier means to the first input of the differential amplifier means; and a fourth resistor means and a fourth capacitor means coupling the voltage reference input of the differential amplifier means to the second input of the differential amplifier means. 13. A method for connecting and conditioning an audio signal comprising: inputting the audio signal; creating a first input resistive drop on one line of the inputted audio signal; and subtracting out common mode noise from the audio signal by using a differential amplifier, the differential amphfier having an output and a voltage reference input. 14. The method as claimed in Claim 13 further comprising: creating a second input resistive drop across two lines of the inputted audio signal. 15. The method as claimed in Claim 13 further comprising: coupling a first line of the inputted audio signal to a first input of the differential amplifier using a first resistor and a first capacitor; and coupling a second line of the inputted audio signal to a second input of the differential amplifier using a second resistor and a second capacitor. 16. The method as claimed in Claim 13 further comprising: coupling the output of the differential amplifier to a first input of the differential amplifier using a third resistor and a third capacitor. 17. The method as claimed in Claim 13 further comprising: coupling the voltage reference input of the differential amplifier to a second input of the differential amplifier using a fourth resistor and a fourth capacitor. 18. A method for connecting and conditioning an audio signal comprising: inputting the audio signal; creating a first input resistive drop on one line of the inputted audio signal; creating a second input resistive drop across two lines of the inputted audio signal; coupling a first line of the inputted audio signal input to a first input of a differential amplifier using a first resistor and a first capacitor, the differential amphfier having an output and a voltage reference; coupling a second line of the inputted audio signal input to a second input of the differential amplifier using a second resistor and a second capacitor; coupling the output of the differential amplifier to the first input of the differential amplifier using a third resistor and a third capacitor, and coupling the voltage reference input of the differential amplifier to the second input of the differential amplifier using a fourth resistor and a fourth capacitor.

Inventors: HEYL LAWRENCE FREDERICK, HEYL, LAWRENCE, FREDERICK
Applicants: APPLE COMPUTER, APPLE COMPUTER, INC.
Application year: 1994
Main classification: H03F3
IPC codes: H03F3